Basics of Phaco - 3
We shall now focus on Phaco Energy or the power applied to break the nucleus for removal of the quadrants easily. Longitudinal phaco power is a factor of two things largely – stroke length and frequency. While there are other factors like the bevel ( orientation of the opening of the tip ) also contributing to the effectiveness of the energy delivered into the eye, let us concentrate on the stroke length and frequency, as these are the two most important determinants of the effectiveness of the energy delivered into the eye.
Before we look into the factors mentioned here, please do not forget to remember the three basic principles of phaco that we started our series on Basics of Phacoemulsification with. One of those principles are less corneal heat buildup, particularly on the incision site. Hence increasing the stroke length and frequency of the hand piece may result in delivery of more punch power to break the nucleus, but it may be contrary and acting against the third principle of phaco, that is, building more heat on the incision site thus creating a chance of thermal injury to the cornea.
What is stroke length ? Stroke length is the distance covered by a tip inside the eye. How far the phaco tip would travel at a selected percentage of power. The Alcon longitudinal phaco typically travels 90 microns at 100 percentage of power. The J&J phaco machines ( formerly AMO) would cover 4.6 mils at 100 percentage of power. The Stellaris phaco machine would cover more than 5 mils at a similar percentage of power.
When you enter the OR and see the staff nurse increasing the power in the panel, he/she are simply increasing the distance being travelled by the tip inside the eye. More the distance travelled, higher is the force applied to break the nucleus. However you cannot go on increasing the stroke length beyond a certain amount as this may lead to a generation of more heat on the cornea leading to a severe complication and endothelial damage.
The second component is frequency. Frequency is the number of cycles made by the phaco tip in one second. The term ultrasound is a misnomer in phaco. We do not deliver ultrasound in the eye. It is referred to as Ultrasound because the tip moves at ultrasonic speed in the eye. That is the number of cycles that the phaco tip makes is typically around 40000 hz or 40 khz. That is why you cannot see the tip vibrate or hear the sound of the tip moving. Note that the frequency or number of cycles a tip would make is fixed. That is determined by the manufacturers. The surgeon can only increase or decrease the stroke length, that is the distance a phaco tip would cover in the eye, by manually increasing or decreasing the power in the GUI ( Graphic User Interface ) or screen.